I have a question about the recipe "Meyer Lemon Macarons" from DolcettoConfections. can i use a plastic bag with the tip cut to pipe the dough? i only have a size 12 piping tip. and if so how large should i cut the slip in the plastic bag?

You can absolutely do that. Just this week my tip fell down the disposal and I had no choice but to use a bag with a cut tip. I much prefer a metal tip, but the disposable pastry bag or even a ziplock will work too.